1. Field of the Invention
The present invention relates to an apparatus for manufacturing semiconductor devices. More particularly, the present invention relates to a wafer pre-alignment stage and to apparatus for controlling the temperature of a wafer held by a wafer chuck of the pre-alignment stage.
2. Description of the Related Art
Generally, semiconductor devices such as semiconductor chips are manufactured by subjecting a wafer made of silicon to a series of semiconductor manufacturing processes such as lithography, light exposure, ion implantation, chemical and mechanical polishing, chemical and physical vapor deposition, and plasma etching or the like.
The light exposure process typically comprises wafer alignment and UV-light exposure steps. More specifically, a wafer coated with photoresist is positioned on a wafer stage in proper alignament with a mask, and then the wafer is exposed to UV-light to form a photoresist pattern.
Such a light exposure process and attendent apparatus are disclosed in U.S. Pat. No. 5,706,076 (issued to Minoru Takeda on Jan. 6, 1998), U.S. Pat. No. 5,781,277 (issued to Kazunori Iwamoto on Jul. 14, 1998), U.S. Pat. No. 5,526,093 (issued to Kazhiro Takahachi on Jun. 11, 1996), and U.S. Pat. No. 5,842,824 (issued to Kenji Nishi on Dec. 1, 1998).
The wafer alignment process mentioned above generally consists of a wafer pre-alignment step which is carried out on a wafer pre-alignment stage and a main wafer alignment step carried out on a main wafer stage. In the wafer pre-alignment step, the position of a wafer is determined by both an edge sensor which detects the edge of the wafer and by an alignment mark sensor which detects an alignment mark on the wafer.
Once pre-aligned, the wafer is then transferred to the main wafer stage. There, the position of the wafer is accurately detected by a number of minute detecting sensors. The wafer is positioned in final alignment with the mask based on the position detected by the sensors.
FIG. 1 is a schematic-plan view of a conventional light exposure system 500. As shown in FIG. 1, the light exposure system 500 includes a wafer transfer stage 510 for storing a wafer W, a wafer pre-alignient stage 520 for pre-aligning the wafer W transferred by the wafer transfer stage 510, and a main wafer stage 530 for precisely aligning the wafer W transferred from the wafer pre-alignment stage 520. The wafer pre-alignment stage 520 is positioned between the wafer transfer stage 510 and the main wafer stage 530.
The wafer W on the wafer transfer stage 510 is coated with a photoresist. The photoresist coating is performed by a spinner executing a spin-coating method. The wafer W coated with the photoresist is transferred to the wafer transfer stage 510 by a first handler 512 once the coating process is completed.
The wafer pre-alignment stage is provided with second and third handlers 522 and 532. The second handler 522 functions to transfer the wafer W from the wafer transfer stage 510 to the wafer pre-alignment stage 520, and to return the wafer W from the wafer pre-alignment stage 520 to its original position on the wafer transfer stage 510 after the wafer W has been subjected to a light exposure process.
The third handler 532 functions to transfer the wafer W from the wafer pre-alignment stage to the main wafer stage 530, and to return the wafer W at the completion of a light exposure process from the main wafer stage to its original position on the wafer pre-alignment stage 520. A pivotal movement of the third handler 532 over a range of 120 degrees enables the third handler 532 to return the wafer W to its original position on the wafer pre-alignient stage 520.
The wafer pre-alignment stage 520 is also provided with an edge sensor 550 and an alignment mark sensor 560. The edge sensor 550 is used to detect a flat-zone of the wafer W and includes a light emitting device (not shown) and a light receiving device (not shown). Because the alignment mark sensor 560 detects an alignment mark formed on a side portion of the wafer W after the flat-zone has been detected by the edge sensor 550, a fine and accurate pre-alignment can be achieved in the wafer pre-alignment stage 520.
The main wafer stage 530 is provided with a number of light detecting sensors 534 which are used to detect the position of the wafer W on the main wafer stage 530. The detected position is then output to a controller (not shown) which activates a wafer positioning device (not shown) based on the information outputted by the light detecting sensors 534 to finely control the position of the wafer W on the main wafer stage 530. Due to such fine alignment processes carried out by the wafer pre-alignment and wafer main stages, thin films can be formed on the wafer-W without the occurrence of an overaly. Here, the term overlay refers to a case in which a second thin film is deposited on the surface of a first thin film while being offset from its intended overlying position relative thereto.
After being aligned in the main wafer stage 530, the wafer W is exposed to UV-rays radiating from a UV-generating device 536 to form a photoresist pattern on the upper surface of the wafer-W. In this exposure process, a mask is interposed between the photoresist and the UV-generating device 536.
The operation of the conventional light exposure system 500 will now be described in detail.
First, the wafer W coated with a photoresist by the spinner 540 is transferred to the wafer transfer stage 510 by the first handler 512.
The wafer W awaits awaits processing in the wafer transfer stage 510. Once its turn arrives, the wafer W is then transferred to the wafer pre-alignment stage 520 by the second handler 522. The wafer W is placed in the wafer pre-alignment stage 520 in a position between the alignment mark sensor 560 and the edge sensor 550. Subsequently, the wafer W is rotated at a predetermined speed by a rotating device (not shown) until a flat-zone has been detected by the edge sensor 550.
Once the flat-zone has been detected, the alignment mark sensor 560 detects an alignment mark formed on the wafer W. During the detection process, the wafer W is rotated in minute angular increments until the alignment mark has been detected. At the completion of the pre-alignment process, the wafer W is then transferred to the main wafer stage 530 by the third handler 532.
At the main wafer stage 530, the light detecting sensors 534 detect the position of the wafer W. The light detecting sensors 534 are minute sensing devices which continuously send electric signals, corresponding to the position of the wafer W to the controller. Based on the signals received, the controller activates the wafer positioning device to finely adjust the position of the wafer W to a predetermined position on the main wafer stage 530.
After the completion of the aligning process in the main wafer stage 530, the controller sends an electric signal to the UV-generating device 536 commanding the UV-generating device 536 to irradiate the wafer W. In the irradiating process, a mask having a predetermined pattern is interposed between the photoresist and the UV-generating device 536 to cause respective portions of the polymer in the photoresist formed on the upper surface of the wafer W to become soluble and to be left non-soluble.
Then the wafer W is transferred to the wafer pre-alignment stage 520 by the third handler 532. The wafer W is then returned from the wafer pre-alignment stage 520 to its original position on the wafer transfer stage 510 by the second handler 522.
Thereafter, the wafer W is transferred to a subsequent stage to be cleaned with a developer for removing the soluble polymer portion of the photoresist and thereby produce the same pattern that was on the mask.
The conventional light exposure apparatus, such as the light exposure apparatus 500 described above, has a significant drawback in that a relatively high temperature is generated in the wafer pre-alignment stage 520 compared to the wafer transfer stage 510. This causes a physical thermal expansion of the wafer W. Such physical thermal expansion conributes to occurrences of overlay of thin films on the wafer W, in turn creating defects in the resultant semiconductor devices.
More specifically, while the wafer W is on the wafer pre-alignment stage 520, a portion of its outer periphery is surrounded by the edge sensor 550 which conventionally has a temperature of 35xc2x0 C. With such a relatively high temperature, the edge sensor 550 tends to radiate heat toward a portion of the wafer W. As a result, the temperature of the wafer W becomes uneven over its surface, which non-uniformity in surface temperature can cause an overlay to occur in the thin films deposited on the wafer W.
In view of the foregoing, it is an object of the present invention to provide apparatus for maintaining a uniform surface temperature condition of a wafer in a wafer pre-alignment stage.
In order to achieve the above object, the present invention provides air injection means for injecting cooling air towards a wafer located at the pre-alignemnt stage, and an air guide for guiding a predetermined amount of the injected air towards a portion of the wafer adjacent the edge sensor to compensate for the relatively high temperature condition existing at that region.
The air injection means is positioned above an upper surface of the wafer and is connected with an air supplier, and includes an air injection head having a plurality of injection holes. The air guide is a flow rate controlling plate which divides the interior of the air injection head into a first space portion and a second space portion, the second space portion being located adjacent, i.e. directly above, the region of the edge sensor. The air introduced into the second space portion is guided towards the edge sensor by the flow rate controlling plate.
The flow rate controlling plate may be fixed in place or may be movable so as to allow the volumes of the first and second space portions to be varied.
An air duct is connected to an air supplier at its proximal end. The distal end of the air duct extends towards the upper surface of the wafer. A neck is formed at the bottom of the distal end of the air duct. The air injection head, in turn, extends from the bottom of the neck. The air injection head has a bottom plate through which the plurality of injection holes extend.
The air injection head is interposed between the edge sensor and the alignment mark sensor and its overall outer shape is that of a cylinder having a diameter larger than that of the wafer which is to be aligned at the stage. The air injection head also has a recessed outer peripheral portion for receiving the alignment mark sensor and a cut-away outer peripheral portion in which the edge sensor is accommodated without contacting the air injection head.
An upper wall portion of the flow rate controlling plate is located inside the neck and divides the interior thereof into two equal regions such that equal amounts of air are introduced through the neck into the first and second space portions. A lower wall portion of the flow rate controlling plate is inclined by a predetermined angle so as to extend in a direction towards the edge sensor, thereby directing the flow of air from s the second space portion towards the region of the edge sensor.